Set encoding to fix pod2man errors.

```
POD ERRORS
       Hey! The above document had some coding errors, which are explained below:

       Around line 307:
           Non-ASCII character seen before =encoding in '2Ï€'. Assuming CP1252
```
Use UTF-8 for '2π'.

lib/PDL/Graphics/Gnuplot.pm

@@ -69,6 +69,8 @@
 # PDL::Options in other circumstances.
 #
 
+=encoding UTF-8
+
 =head1 NAME
 
 PDL::Graphics::Gnuplot - Gnuplot-based plotting for PDL